I. Field of the Invention
The present invention relates to a lockout arrangement for an inverter circuit, and more particularly, to an emergency lighting circuit including an inverter for operating a gaseous discharge lamp from a DC electrical energy source upon failure of the primary AC source, the circuit being provided with means for shutting down inverter operation if the lamp is removed from the circuit.
II. Description of the Prior Art
Electric power failures due to inclement weather conditions and equipment breakdowns have been a plague for many years. A power failure, no matter what the cause may be, may very well jeopardize human life due to lighting system failure. There are, therefore, many installations which require some type of emergency lighting system which will automatically come into operation upon the occurrence of a power failure. The high efficiency of a fluorescent lamp makes it especially valuable for use in such an emergency lighting system.
Presently available emergency lighting systems are generally of the type using a transistor switching inverter and wherein a single lamp or group of lamps is used for both normal AC operation of the lighting system and for the emergency system, a battery being used as the power source for energizing the transistor inverter and the lamps when the AC line voltage fails. A system designed especially for use with fluorescent lamps which are eight feet in length or longer, must be designed to reliably hot spot and operate such lamps under an assortment of adverse conditions. Such conditions include degraded or partially discharged batteries, old lamps, and after long periods of operating the battery at high temperatures. Therefore, when the system ages, the inverter must still be capable of generating a high voltage and appreciable power in order to reliably start the lamp. When conditions are optimum, such as at the beginning of the system's life, the inverter is capable of generating an even higher voltage and more power. In such a condition, if the lamp is removed from the socket, the combination of stray capacitance in the fixture, the power system, and the inverter furnish potential high frequency return paths and extreme safety hazards should a person contact exposed lamp terminals or lamp socket terminals.
It is desirable, therefore, to provide an emergency lighting system including an inverter circuit wherein such potential electrical shock hazards are obviated.
Accordingly, it is an object of the present invention to provide an emergency lighting system having an inverter wherein means are provided for shutting down inverter operation when a lamp is removed from its socket in the fixture allowing the circuit to be placed in an unloaded condition.